Added since the RadeonHD 1.2.3 driver in October is HDMI audio support, support for the ATI RV710 and RV730 GPUs using DCE 3.2, rotation support, RandR 1.3 panning, and many acceleration and build fixes. Click on any of the above links for more information on the individual features.
While on the topic of an open-source ATI, there still is no sign on when there will be open-source R600/700 3D support added to the Mesa stack, but it's starting to look like we won't see it this year and perhaps not until FOSDEM 2009 in February. The code and documentation is still stuck in its legal review process.
The release announcement for the RadeonHD 1.2.4 DDX driver can be read on the mailing list.